The isocyanates are an important class of commercial chemicals used in the production of polyurethanes and certain pesticides. In addition, various forms of isocyanate compounds are emitted during the thermal decomposition of urethane materials. Isocyanates are documented sensitizers, causing occupational asthma, hypersensitivity pneumonitis, and accelerated annual decline in lung function. The compounds are characterized by a high degree of chemical reactivity, and some have a high for surfaces and relatively low vapor pressures at ambient temperatures. These properties result in a tendency for some isocyanates to partition into gaseous and particulate phases in the atmosphere. The physical state of the compound will affect its site of deposition/adsorption in the respiratory tract, as well as dictate the appropriate sampling techniques and engineering control methods. This project entails the development of a dichotomous sampler for total reactive isocyanate group (TRIG) vapor and particulate consisting of an annular diffusional denuder for collection of the vapor phase, backed up by a treated filter for collection of the condensed phase. Analysis of collected TRIG will be by HPLC. Three common derivatizing agents (methyamino-methylanthracene, methoxyphenyl-piperazine, tryptamine) will be evaluated for reliable determination of TRIG using a series of thirteen model isocyanate compounds. The reagent exhibiting the most consistent analytical response to the model isocyanates will be utilized in the dichotomous sampler. The sampler will be optimized in terms of its critical length and sampling flow rate, and its performance characteristics will be thoroughly evaluated using test atmospheres of aerosol and vapor phase isocyanate, and compared to impinger and filter sampling methods. The sampler will be used to characterize TRIG in workplace atmosphere at two polyurethane foam plants and two isocyanate manufacturing plants. This work will result in a more thorough understanding of the behavior of TRIG in the atmosphere, and will offer a powerful tool for future studies of the potential differences in the biological activities and behaviors of vapor and condensed phase isocyanate.